As known by the man skilled in the art there is an increasing demand for a high transmission reach in optical access networks, typically 40 km or even 60 km, in order to get downstream and upstream nominal line rates per channel equal to 10 Gb/s and 2.5 Gb/s or 10 Gb/s respectively.
There is also a demand for high dynamic extinction ratio (or DER), typically 6 dB or even 8 dB.
Several solutions have been proposed to reach at least one of the above mentioned characteristics.
A first solution consists in carrying out a proper management of chirp and spectrum reshaping. This first solution is rather well adapted to reach intermediate distances (typically between 100 km and 300 km) and/or to allow reducing the spectral broadening through dual modulation. But it induces a DER that is too low.
A second solution consists in using an electro-absorption modulator (or EML). But it induces a loss of optical power due to the absorption into the modulator. To improve the situation it is possible to use a passive taper section and to grow different materials for the laser and the modulator. But this increases the technology complexity and leads to a high consumption 3-sections device.
A third solution consists in using an integrated chirp managed laser (or CML) with an optical spectrum re-shaper to increase dispersion tolerance, However, the targeted transmission distance ranges (from 200 km to 600 km) are far beyond the optical access network standards. Beside, this solution makes necessary to precisely tune the laser wavelength to the optical spectrum re-shaper characteristic, which requires the use of a complex feedback loop that makes it viable for very long distance networks but unsuitable for low cost applications.
A fourth solution consists in using a transmitter optical sub-assembly (or TOSA) with an hybrid integration of directly modulated lasers (or DML) and free-space-optics assembly for signal spectral filtering. This solution fully fits to the distance transmission and DER requirements, but its main drawback is the complexity of the module packaging since a precise alignment of the on-wafer laser with the free-space-optics is required, which induces additional coupling losses that could be a limitation for a stringent optical budget recommendation in an optical access network. Moreover, the achieved performances are only possible by means of a complex and power consuming electronic dispersion compensation (or EDC) system.
A fifth solution consists in using a planar ligthwave circuit (or PLC). This allows achieving a 300 km transmission at 10 Gb/s without any optical or electrical dispersion compensation. However, the fabrication and use appear to be complex since two monitor port photodiodes are requested and high insertion losses are expected from the hybrid integration of III-V semiconductor based laser on silicon platform.